1. Technical Field
The invention refers to a device for transmitting signals between two terminals, of which at least one is moveable with respect to the other, and which are connected by at least one conducting line wound in turns. Other conducting lines can be connected between the two terminals. The line is stored in an essentially circular cassette comprising a stator supporting one of the terminals and a rotor which rotates around its axis and with respect to the stator and supports the other terminal. The turns of the line are subdivided into an inner winding area and an outer winding area with different winding directions, between which an approximately U-shaped reversing area is located. A ring-shaped guide encompasses the reversing area of the turns is located between the two winding areas and can move in the peripheral direction of the cassette. At least the internal periphery of the ring-shaped guide is configured to receive the turns of the inner winding area.
Such a device is needed for example to transmit a signal for releasing the impact protection "airbag" in motor vehicles. It is located in the steering wheel of a motor vehicle for transmission of an electrical or optical signal. In the sense of the invention, the "line" can therefore be an electrical or an optical fiber line. A significant problem toward this device is the transmission signals between fixed and moveable parts of the motor vehicle. The sliding contacts or sliding rings that transmit current, which have been known a long time for such instances, are subject to wear and are particularly disadvantageous at low power due to the fluctuating transition resistances.
2. Description of the Prior Art
An electric current is used for the transmission of signals in the known device according to EP-OS 0 417 350. The transmission of current takes place through a flat ribbon line or cable wound by a kind of spring barrel into a winding body and stored in a cassette. During a relative rotating movement of the two terminals connected by the flat ribbon line, the wound flat ribbon line "breathes" like the spring of a watch. In one of the directions of rotation, the turns of the wound flat ribbon line are drawn together into a smaller diameter. In the other direction of rotation, they increase again into a larger diameter. In this way, the flat ribbon line can participate in the rotating movements of a steering wheel without interrupting the signal path. To be able to perform a sufficient number of rotations in both directions, a correspondingly long flat ribbon line is needed, which is found in a large number of turns in the cassette. In principle, this device can be used in practice. However, because of the long length of the line and its relatively high ohmic resistance, difficulties can occur with the signals to be transmitted, particularly in the presence of low currents.
In the device described in DE-OS 42 11 264, the length of the line between the two terminals is significantly shorter than in the above described device. The wound line is divided into inner and outer winding areas, in which the windings turn in different directions. The two winding areas are connected with each other by an approximately U-shaped reversing area. A ring-shaped guide is located between the two winding areas and surrounds the reversing area. The guide can move in the peripheral direction of the cassette. It is moved by the wound line or by its reversing area when the cassette rotor turns. The guide contains several rollers, which are offset with respect to each other in the peripheral direction and rotate respectively around an axis, to which one line each leads from the inner winding area and from the outer winding area when the rotor turns. The rollers prevent the inner winding area from resting too closely against the guide, which would jam it. The guide in this known device is expensive, especially because of the separate location of each roller.